Plastic container with decreased gas permeability

ABSTRACT

A container made of an organic resin having improved vapor barrier characteristics is disclosed which achieves an improved barrier by the placement thereon of a thin coating of an inorganic material.

BACKGROUND OF THE INVENTION

The present invention relates generally to plastic containers and moreparticularly discloses containers, such as bottles and cans, havingimproved gas transmission barrier characteristics.

In the food and beverage industry the trend is to move away frompackaging perishable products in glass and metal containers and tosubstitute thermoplastic polymers for the container material. One of themost successful polymers for beverage containers to package beer, wine,and soft drinks has been polyethylene terephthalate (PET). One of thelargest markets for PET containers has been in the two-liter carbonateddrink field. Another area where PET is expected to be used extensivelyis in packaging beer and food. In either case, one of the most criticalcharacteristics of the polyester package is the prevention of gaspermeation through the wall of the container.

With carbonated soft drinks, the problem with gas permeation is the lossof carbonation (CO₂ gas) from the drink through the wall of the bottleor can. Compared to the small, densely-packed metal and glass molecules,polymer molecules are relatively large and form a porous wall. Even thebest polymer known at this time for gas barrier properties, ethylenevinyl alcohol (EVOH), has poor barrier ability when compared to theinorganics such as metals and glass.

On the other hand, beer and food containers preferably should present agood vapor barrier against the ingress of oxygen (O₂) into the containerbecause of the accelerated spoilation of the food products caused by thepresence of oxygen therein.

While the use of PET two-liter containers has been relativelysuccessful, its use in smaller-sized containers such as half-liter andone-third-liter, is very limited because of the greatersurface-area-to-volume ratios of the smaller containers, compared tothat of the two-liter container. This proportionate increase in thesurface area causes a much more rapid loss of carbonation from and/oringress of oxygen into the containers and thus decreases the"shelf-life" of the contained product.

There have been several different methods developed in an attempt toincrease the "shelf-life" of plastic containers. One of the most commonmethods involves creating a multi-layered container having a thinbarrier layer of a material such as EVOH or polyvinylidene chloride(PVdC) buried between two or more layers of a container polymer such asPET, polypropylene, polystyrene, or PVC. This multi-layer container isdifficult and expensive to manufacture since the barrier layers areeither expensive (EVOH) or corrosive (PVdC). Also the process forforming a multi-layered material and making a container from it may bemuch more complex than single-layer processes.

Another method of creating a barriered polymer container is the processknown as "dip-coating". In this process a polymer bottle made of amaterial such as PET, is first formed into its final shape and then theadditional step of dipping the container into a coating solution isperformed. This solution may be of a barrier material such as PVdC. Thisprocess, in addition to adding another expensive step to the containermanufacture, also introduces a material to the container that preventseasy recycling. Because of the nature of PVdC, the coating must beremoved by solvents before the polymer container can undergo normalrecycling. In light of the trend toward compulsive container return lawsin various states and a probable federal deposit/return law, all futurecontainer designs must be quickly and easily recyclable. Dip-coatedbottles do not lend themselves to easy recycling.

The present invention overcomes the deficiencies of the barrier-layercontainers and the dip-coated containers by providing a barrier-treatedplastic container which provides excellent barrier characteristics, ischeaply and easily treated, and can be completely recycled byconventional recycling techniques without need for removal of dip-coatedlayers.

This is achieved by impregnating the surface of a normal polymercontainer with an inorganic material such as a metallic oxide. Theimpregation is done by gasless ion plating to provide an ultra-thinflexible coating of the inorganic material on the plastic substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the process of the present invention.

FIG. 2 is a magnified cross-sectional schematic view of a coated polymermaterial.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the present invention, a number of one-half literpolyester bottles (polyethylene terephthalate) were placed in a vacuumchamber and a vacuum of about 1×10⁻⁵ Torr was drawn on the chamber. Aplating source comprising silicon monoxide (SiO) was vaporized into ametallic oxide vapor. The vapor was ionized into a plasma by an RFenergy source and then biased by a DC bias to impinge the substrate(bottle) surface with a sufficiently high energy level to penetrate theSiO ions partially into the substrate polymer. This process is generallythe same as that disclosed for metallic and non-metallic substrates inU.S. Pat. Nos. 4,016,389, 4,039,416, Re 30,401, and 4,342,631, whichpatents are hereby incorporated by reference into this application.

Referring now to FIG. 1, which is a schematic illustration not drawn toscale, disclosed is a vacuum chamber 10 having a substrate holder 11removably secured therein. At least one plastic bottle 12 is looselyheld on the portable fixture 11 below a source of inorganic material 15held in a vaporizing filament 13. Filament 13 is electrically connectedto and supported by a pair of terminals 14. It preferably is aresistance heating element powered by an external AC power supply (notshown). As the coating material 15 held in filament 13 is vaporized bythe filament, an ionizing energy, comprising RF (radio frequency), and abiasing DC voltage, are placed on the filament 13 with respect to thesubstrate holder 11 which is grounded.

As a result of the vaporization of the source material and the ionizingand biasing field created by the DC/RF power supply, a plasma of ionizedSiO particles forms between the filament 13 and the substrate holder 11.The bias also accelerates the SiO ions toward the fixture 11a which islocated inside the bottle 12. The ions impinge the outer surface of thepolyester bottle while traveling at very high velocities and apparentlyeven penetrate partially into the surface of the polymer. An evencoating can be obtained by rotating the bottle 12 about one or more ofits axes during the impingement cycle.

The impingement cycle is maintained long enough to obtain a coatinglayer of around 500 angstroms thickness. The result is a clear flexiblecoating of SiO on the outer surface of the polyester bottle, which it isbelieved actually penetrates partially into the polymer and plugs theinterstices and porosities between the polymer chains. This plugging ofthe interstices is believed to be a main contributor to the improvementin gas barrier characteristics of the SiO coated container. While 500Angstroms is considered a good coating thickness, other thicknessesranging from less than 500 to as high as 5000 Angstroms or more might beused depending on the type of polymer, the container shape, and the sizeand thickness of the container.

For example a series of PET half-liter bottles were ion-plated with SiO,and the measured CO₂ transmission rate was reduced from ##EQU1## Apressure test was performed over a period of time to determine ifflexure during filling or stretching under pressure by the containercaused degradation or flaking off of the inorganic material. It wasobserved that flexure and creep did not significantly degrade thebarrier characteristics. Manual flexure of several containers was alsoperformed to test for cracking or flaking of the coating. After thesesteps were performed, acetone was applied to the coated surface todetect any breaks in the integrity of the coating. Since acetone willnot attack inorganics like SiO₂ but is a strong solvent for the polymer,any break in the SiO coating would have allowed the acetone to attackthe container polymer. No dissolving of the container was observed afterapplication of the acetone to the outer surface. Thus flexure and creepnot only had no effect on the barrier properties they also had nodetrimental effect on the surface continuity of the coating.

In addition to the impingement coating of polyesters such as PET it isbelieved that most other polymers can also be coated successfully. It isalso expected that other inorganic materials may be substituted for SiO,for example aluminum oxide and titanium oxide. Most inorganic ormetallic oxides should be adaptable to this process. It should also benoted that even though the metals of these plating compounds aregenerally opaque, their oxides are clear and thus they can be used onboth clear and pigmented polymers without affecting the aesthetics ofthe containers.

Recyclability of the used coated containers is not affecteddetrimentally because of the extremely small amount of inorganic coatingused. Because of its inert nature and presence in small amounts, thecoating will not be noticeable in the recycled polymer. The amount ofinorganic coating is less than 1% by weight of the polymer in thecontainer. Some containers have inorganic pigments such as titaniumdioxide mixed with their polymers in amounts as high as 25% by weightwithout affecting recyclability; therefore it can be seen how negligiblethe effect of the coating material is on recyclability using the presentinvention.

Referring now to FIG. 2, there is illustrated a schematic enlargedillustration of a section of the coated surface indicating how it isbelieved that the present invention increases barrier properties. In thedrawing, which is not an attempt to show true scale, polymer molecules Pare shown having long, winding structure which when combined togetherresult in large openings therebetween. Inorganic metalic oxide moleculesM, such as Silicon Oxide or Aluminum Oxide, are very small and compactand can be infiltrated into the interstices formed by the long bulkypolymer molecules. Because of the vacuum environment around thesubstrate and the high velocity of impingement during the plating, theinorganic molecules can penetrate deeply into the polymer interstices,giving good binding between the coating and the substrate. A very thinlayer, on the order of around 500 Angstroms, of the metallic oxide isapplied to the substrate, giving a good barrier in the interstices andhaving sufficient flexibility to withstand breakage when flexed.

Although a specific preferred embodiment of the present invention hasbeen described in the detailed description above, the description is notintended to limit the invention to the particular forms or embodimentsdisclosed therein since they are to be recognized as illustrative ratherthan restrictive and it will be obvious to those skilled in the art thatthe invention is not so limited. For example it is contemplated thatplating materials other than silicon monoxide, aluminum oxide, andtitanium oxide can be used. One such material would be tantalum oxide.Also containers other than carbonated beverage bottles would benefitfrom the present invention, such as beer containers, food containers,and medicine containers. Thus the invention is declared to cover allchanges and modifications of the specific examples of the inventionherein disclosed for purposes of illustration, which do not constitutedepartures from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A container havingdecreased gas permeability through the walls thereof, said containercomprising:a thermoplastic organic resin container having at least onesidewall portion and a bottom portion made of said resin; and, a thinlayer of an inorganic oxide coating material ion-deposition plated on atleast part of said sidewall portion.
 2. The container of claim 1 whereinsaid resin is a polyester polymer and said inorganic coating is a metaloxide.
 3. The container of claim 1 wherein said inorganic coating issilicon monoxide.
 4. The container of claim 1 wherein said resin is apolyolefin and said coating is an inorganic oxide selected from thegroup consisting of tantalum oxide, titanium oxide, silicon monoxide andmetallic oxides.
 5. The container of claim 1 wherein said coating is upto about 500 Angstroms thick.
 6. A polymer container having relativelythin walls and a reduced gas permeability therethrough, said containercomprising:a structural body portion made of an organic resin polymer;and, a coating on said polymer comprising an inorganic oxide ion-platedthereon.
 7. The container of claim 6 wherein said inorganic oxide isselected from the group consisting of silicon monoxide, tantalum oxide,and metallic oxides.
 8. The container of claim 6 wherein said inorganicoxide is applied to said container by gasless ion plating.
 9. Thecontainer of claim 8 wherein said inorganic oxide is up to about 500Angstroms average thickness on said container.
 10. A thermoplasticpolymer container having decreased gas permeability through the wallsthereof, said container comprising:a body portion having an enclosedbottom portion, all made of a thin polymer selected from the groupconsisting of polyolefins and polyesters; and, an inorganic oxidecoating deposited on said body portion by gasless ion plating in a layerof up to about 500 Angstroms thick.
 11. The container of claim 10wherein said coating is selected from the group comprising siliconoxides, tantalum oxides, and metallic oxides.
 12. The container of claim11 comprising a bottle made of polyethylene terephtahalate and a coatingof an inorganic oxide.
 13. The container of claim 11 comprising a bottleof polypropylene and a coating of an inorganic oxide.
 14. The containerof claim 12 wherein said oxide is silicon monoxide.
 15. The container ofclaim 13 wherein said oxide is silicon monoxide.